This grew out of reading various discussions online concerning shows, books etc that have angels, in whatever form, and I wondered how Islington would fit into the Supernatural 'verse, and Castiel within Neverwhere. In the end, it was simpler to bring Cas into Neverwhere, although I made one change in referring to the angels as 'he' rather than 'it' as Neil Gaiman does, (partly because Castiel is generally considered male within Supernatural and partly because using 'it' all the time is a pain in the ass when it comes to constructing sentences).

I should also note that my interpretation of Islington is influenced by Peter Capaldi's performance in the TV version, for those who've only read the book.

Repentance

There was darkness. There was silence.

There was the Angel, sitting alone.

There was nothing else.

For the first few hundred years or so, the null had been filled with tremendous screams, the raging and fury of a divine being tricked by a mere mortal, one little human in a Universe of tiny, insignificant people. Then, after a few centuries of venting, Islington began to calm, to realise that there was nothing that could be done, trapped as he was within this nothing.

It was, after all, not even the place the Opener bitch had sent him. That had been easy to escape; a simple little nightmare vale that had served to remove his two irritating former associates before Islington had moved on.

But in order to continue escaping from the places he found himself in, he had been forced to pass through null space and found himself trapped once more.

After immeasurable years imprisoned within the vault of London Below, denied access to Heaven and his fellow angels, he could bear captivity no longer – to have come so close.

But trapped he was and there was nothing he could do about it. Time would pass differently here; to him, it could be centuries, to those on the outside, mere moments.

And as it passed, the angel began to think, and those thoughts were not those he had had since the sinking of Atlantis; the dreams of returning to Heaven to conquer it, of all of Creation yielding to Islington, of power beyond what any filthy little mortal mind could comprehend.

For some reason, one the angel couldn't name, he no longer desired this. Even the thought of revenge, of taking apart those who had thwarted him, piece by tiny screaming piece, of blasting them into non-existence, no longer held the same appeal, which surprised Islington. Before, he would have thought that nothing could divert him from his path, and that if something did, then revenge was something he would never give up on, but perhaps the complete absence of anything here in the void was changing the angel.

His prior imprisonment had been a punishment, one he had railed against, seen as an injustice; what fault was it of his if a few million inconsequential little souls were lost when there were so many more to replace them? This time, it had a different effect on Islington, and he felt himself begin to calm.

He was old, could no longer remember what it meant to be young, what it had been like to exist within Heaven's domain and he ached for it; not as a prize to be won or stolen, but simply to go home. To be again what he had been created to be, whether the Creator remained or no.

And in the meantime, because there was nothing to do, he did nothing.

But after a while (probably no more than seven millennia or so), Islington began to find that there were things he could still do. He could not, strictly, create anything, the null would not permit it, but he found that light escaping from his fingertips lingered in the black for a while before dissipating, and he found himself using this evanescent luminosity to draw, using nullspace as canvas. In this fashion, he recreated fleeting landscapes, and transient seascapes that had not been seen by human eyes for centuries, even before he had come to this dark land of nothing. He drew faces that were not human, had not existed since long before humankind began to drag itself out of the primordial oceans of Earth, but that remained in Islington's mind, and this activity brought a certain peace, almost contentment. That was not so say he did not still yearn for home, but it lessened the ache somewhat and made the isolation, the nothing a little more bearable.

And then, all at once, Islington was not alone within the null anymore.

Truth be told, there were no signs to perceive; no sound, no movements but first he was alone, then he was not.

"Hello, Castiel."

"Islington."

His brother angel was standing behind where Islington was sitting, appearing from nowhere into nowhere as if it were no challenge at all.

"I'm surprised that they sent you to collect me. You were always so eager a soldier, so keen to fulfil your tasks. Something as minor as prisoner detail…"

"I have not been sent. I came here of my own free will."

Islington rose to his feet in a fluid motion, his white robe swirling around him.

"Free will? There was a time, brother, I would never have believed I would hear those words leave your mouth."

"There have been… changes. On Earth as well as within Heaven."

"I'm sure there have. As you can imagine, I receive no news here."

"You were not easy to find."

Islington turned to face his brother. Eyebrows raised in surprise at what he perceived.

"You came here in a human vessel?"

"It seemed the best way to avoid becoming trapped within the null."

"Ingenious. Even had I not been locked within this form, I would not have thought of that."

Islington took a step closer, his head tilted to one side as he looked closer into the blue eyes that acted as windows onto his brother's true form.

"You are a great deal altered. How long have you held this shape?"

"A while. I have… become used to it."

"While walking the Earth. Our Father sent you on an errand?"

"I was tasked with the aid and protection of Michael's vessel, in the forerunning to the Apocalypse."

Castiel's expression did not waver as he imparted this information, although within, he knew that this simple phrase covered the most tumultuous phase of his existence.

Islington's face in response, however, was a picture.

"I missed the Apocalypse?"

"No. It was prevented. Sort of."

"Explain."

"It is too complicated. But I will say that we have all underestimated humans, and the role that they play in the Plan."

"And now the Apocalypse has been averted, you have come to take me home?"

There was a plead in Islington's voice that brought on an all-too-human emotion within Castiel. His – experiences – since taking a vessel had transformed him from the ground up, as his human companions might say, but Islington was not the angel he had once been, either.

"As I said, there have been many changes. Alongside numerous humans, many of our brothers are dead, and it does not seem right to continue to deny you your place among us."

Islington recognised that this information was the cause of much emotional pain to Castiel. He had indeed become used to passing as human; one might almost say he had begun to become one.

"Our brothers?"

"Among the archangels…Lucifer was returned to his infernal prison. Michael fell with him. Among those killed in battle; Uriel. Zachariah. Gabriel."

A flash went across Islington's face as he took this in, and had Castiel seen his brother while he resided in London Below, he would have recognised the expression. But that had been such a long time ago, and Islington found even that long held loathing had no true call on him now.

"If it were possible for angels in Heaven to hate one another, then that would have been Gabriel and I," was all he said.

"But that was long ago now and I – regret that I will never have the chance to reconcile with him."

Castiel, remembering his own battles with Gabriel in Trickster form, doubted that that would have happened, regardless of Islington's long period of repentance. Although it had come as a surprise to Castiel that Gabriel had died in the cause of trying to avert the Apocalypse; slain by their wild brother who had learned no such penitence throughout his own long imprisonment.

Perhaps Lucifer's prison had not been chosen wisely.

"It is time to return to Heaven. Come."

Castiel extended his hand toward Islington, and as he drew him into an embrace, felt the null release its grasp on his brother, and he took him home.